Manufacturers today produce several sizes of fire hydrants for customers to select. Usually customers select the fire hydrant size based on some form of capacity criteria and they choose one hydrant over another hydrant on the basis of established capacity, and more specifically, head or flow loss. The American Water Works Association as well as Underwriters Labratories provided standards for the industry which ignore hydrant size when spelling out acceptable head or flow loss, these standards grouping together all sizes of hydrants in setting a standard not to exceed a set value of 5.0 pounds per square inch at 1000 gallons per minute. The American Water Works Association standard is identified as standard C502, whereas the Underwriters Labratories standard is identified as standard 246.
Most dry barrel fire hydrants on the market today have head losses which approach 5 psi and usually lie in a range of 3.5 psi to 5.0 psi at 1000 gpm. In this respect, the hydrant's shoe design involves utilizing an elbow-shaped shoe which may or may not have a part spherical chamber therein but when such hydrants do have a part spherical chamber therein, the sphere center is usually located on the intersection of the axis of the inlet passage of the shoe and the axis of the outlet passage and, in some instances, the special chamber has a radius of curvature greater than the radius of the inlet passage of the shoe whereas in other instances the spherical chamber has a radius of curvature equal to radius of the inlet passage of the shoe. Consequently, when the hydrant valve is open and is lowered down into the bowl of the hydrant shoe and the flow of fluid through the inlet passage enters the shoe it impinges on the hydrant valve as well as on the back spherical surface of the shoe where it splits resulting in turbulence within the shoe causing flow losses between the inlet opening of the shoe and the outlet nozzle on the barrel of the hydrant. This situation generally follows for a hydrant that has a conventional elbow-shape without a spherical portion in the shoe bowl or chamber.
More recent designs of fire hydrant shoes have found that some of the head or flow losses of the hydrants can be reduced by providing a part spherical chamber with a spherical center positioned above the axis of the cylindrical inlet passage to the shoe and on the axis of the cylindrical outlet passage from the shoe with the radius of curvature of the part spherical chamber being greater than the radius of the inlet passage and the part spherical chamber merging smoothly with the inlet passage adjacent its lowest portion. However, this type of fire hydrant did not take into consideration the provisions of providing a smooth transitional surface between the entire inlet passage and the part spherical chamber especially in the area where the flow path changes from horizontal to vertical nor providing a smooth transitional surface where flow leaves the shoe chamber around the valve and enters the barrel of the hydrant. The sharp or sudden changes of shape from the cylindrical inlet passage to the larger spherical shape and the sudden or sharp changes of shape from the spherical shape of the shoe to the outlet passage of the shoe still resulted in some turbulence causing flow or head losses in the hydrant. There was no gentle expansion in the flow way in an area where the inlet flow into the shoe impinges on the open main valve and there was no maintaining of a uniform velocity of flow around the open main valve and smooth flow from the shoe into the barrel.
For the most part, hydrants of the prior art did not appreciate that the configuration of the upper surface of the valve element also contributes to head or flow loss in the hydrant. Usually, the valve element was provided with an upper valve plate which was generally flat and thus had no effect in controlling the flow of fluid through and out of the hydrant shoe. Such arrangements resulted in considerable turbulence particularly around the upwardly extending valve stem. However, some valve elements have been made with an upper surface which is generally conically shaped but these valve elements were not utilized in shoe designs which minimized turbulence of fluid flowing into the shoe as well as minimized turbulence of fluid flowing from the shoe around the valve element.